Carbon nanotubes (hereinafter, referred as to “CNTs”) are understood to mean cylindrical carbon nanotubes having a diameter of 3 to 150 nm, preferably 3 to 100 nm, and a length of many times, i.e. at least 100 times the diameter. CNTs consist of aligned carbon atom layers and have different types of cores. CNTs are also called as carbon fibrils or hollow carbon fibers. CNTs are industrially essential in the production of composites because of the size and specific properties thereof and may be utilized in further applications including electrical applications and energy applications.
CNTs are generally manufactured by arc discharge, laser ablation, chemical vapor deposition or the like. However, arc discharge and laser ablation are disadvantageously not suited to mass-production and involve excessive preparation costs or laser purchase costs.
Furthermore, chemical vapor deposition has problems in that synthesis velocity is very low and synthesized CNT particles are extremely small in the case of using a gas-phase dispersion catalyst and there is a limit to bulk production of CNTs because space utilization inside a reactor is significantly reduced in the case of using a substrate-supported catalyst.
The catalytically active component of the catalyst generally has an oxide form, a partially or completely reduced form, or a hydroxide form and the catalyst may be a carbon nanotube catalyst, a co-precipitation catalyst or the like which is commonly used for the production of CNTs. Of these, the carbon nanotube catalyst is preferred because the carbon nanotube catalyst advantageously has a higher bulk density than a co-precipitation catalyst, reduces probability of generation of fine powder by attrition which may be generated during fluidization due to small-amount generation of fine powder of 10 microns or less unlike the co-precipitation catalyst, and enables stable operation of the reactor due to superior mechanical strength of the catalyst.
In addition, as a method for producing a carbon nanotube catalyst, an impregnation method including mixing an aqueous metal solution and a support, followed by coating and drying is suggested. In this case, the produced catalyst has a disadvantage of limited catalyst loading. In addition, heterogeneous distribution of the active component and the catalytic component greatly affects CNT growth yield and CNT diameter distribution, but a method for controlling the heterogeneous distribution has not been suggested to date.
In particular, in accordance with a conventional impregnation method, when carbon nanotubes are synthesized using a prepared supported catalyst, the yield is lower than 1,000% and is limited due to high load. In addition, the carbon nanotubes are a bundle type and thus have low bulk density, decreased reactive gas injection rate and thus reduced CNT productivity.
Accordingly, there is a need for research which is capable of synthesizing carbon nanotubes having a high bulk density at a high yield using a CNT catalyst.